Type II beta-turn conformation of pivaloyl-L-prolyl-alpha-aminoisobutyryl-N-methylamide: Theoretical, spectroscopic, and X-ray studies

Pivaloyl-L-Pro-Aib-N-methylamide has been shown to possess one intramolecular hydrogen bond in (CD3)2SO solution, by 1H-nmr methods, suggesting the existence of beta -turns, with Pro-Aib as the corner residues. Theoretical conformational analysis suggests that Type II beta-turn conformations are about 2 kcal mol-1 more stable than Type III structures. A crystallographic study has established the Type II beta-turn in the solid state. The molecule crystallizes in the space group P21 with a = 5.865 Å, b = 11.421 Å, c = 12.966 Å, beta = 97.55°, and Z = 2. The structure has been refined to a final R value of 0.061. The Type II -turn conformation is stabilized by an intramolecular 4 1 hydrogen bond between the methylamide NH and the pivaloyl CO group. The conformational angles are Pro = -57.8°, Pro = 139.3°, Aib = 61.4°, and Aib = 25.1°. The Type II beta-turn conformation for Pro-Aib in this peptide is compared with the Type III structures observed for the same segment in larger peptides.

The Alpha-Method Direct Transcription In Path Constrained Dynamic Optimization

Numerically discretized dynamic optimization problems having active inequality and equality path constraints that along with the dynamics induce locally high index differential algebraic equations often cause the optimizer to fail in convergence or to produce degraded control solutions. In many applications, regularization of the numerically discretized problem in direct transcription schemes by perturbing the high index path constraints helps the optimizer to converge to usefulm control solutions. For complex engineering problems with many constraints it is often difficult to find effective nondegenerat perturbations that produce useful solutions in some neighborhood of the correct solution. In this paper we describe a numerical discretization that regularizes the numerically consistent discretized dynamics and does not perturb the path constraints. For all values of the regularization parameter the discretization remains numerically consistent with the dynamics and the path constraints specified in the, original problem. The regularization is quanti. able in terms of time step size in the mesh and the regularization parameter. For full regularized systems the scheme converges linearly in time step size.The method is illustrated with examples.

Sulphate ion dynamics in alpha-alums studied by esr at high pressures

The variation of zero-field splitting and linewidth of Cr3+ ion in KCr and KAI alums with hydrostatic pressure and with temperature is investigated. A model for the apparent phase transition is proposed on the basis of the reorientational motion of the SO2�4 groups.

The modes of binding methyl-alpha (and beta)-D-glucopyranosides and some of their derivatives to concanavalin A--a theoretical approach

The probable modes of binding of Methyl--alpha (and beta)-D-glucopyranosides and some of their derivatives to concanavalin A have been proposed from theoretical studies. Theory predicts that beta-MeGlcP can bind to ConA in three different modes whereas alpha-MeGlcP can bind only in one mode. beta-MeGlcP in its most favourable mode of binding differs from alpha-MeGlcP in its alignment in the active-site of the lectin where it binds in a flipped or inverted orientation. Methyl substitution at the C-2 atom of the alpha-MeGlcP does not significantly affect the possible orientations of the sugar in the active-site of the lectin. Methyl substitution at C-3 or C-4, however, affects the allowed orientations drastically leading to the poor inhibiting power of Methyl-3-O-methyl-alpha-D-glucopyranoside and the inactivity of Methyl-4-O-methyl-alpha-D-glycopyranoside. These studies suggest that the increased activity of the alpha-MeGlcP over beta-MeGlcP may be due to the possibility of formation of better hydrogen bonds and to hydrophobic interactions rather than to steric factors as suggested by earlier workers. These models explain the available NMR and other binding studies.

Characterization of a toxin from Parthenium hysterophorus and its mode of excretion in animals

Fractionation of methanolic extracts of air dried aerial parts ofParthenium resulted in the isolation of a toxic constituent which was identified as parthenin, the major sesquiterpene lactone from the weed. The LD50 (minimal lethal dose required to cause 50% mortality) for parthenin in rats was 42 mg/kg body weight. When [3H]-parthenin was given orally or by intravenous administration, radioactivity appeared in the milk of lactating laboratory and dairy animals. Tissue distribution of radioactivity revealed that maximum label was detectable in kidneys.

Parallel packing of alpha-helices in crystals of the zervamicin IIA analog Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-OMe.2H2O.

An apolar synthetic analog of the first 10 residues at the NH2-terminal end of zervamicin IIA crystallizes in the triclinic space group P1 with cell dimensions a = 10.206 +/- 0.002 A, b = 12.244 +/- 0.002 A, c = 15.049 +/- 0.002 A, alpha = 93.94 +/- 0.01 degrees, beta = 95.10 +/- 0.01 degrees, gamma = 104.56 +/- 0.01 degrees, Z = 1, C60H97N11O13 X 2H2O. Despite the relatively few alpha-aminoisobutyric acid residues, the peptide maintains a helical form. The first intrahelical hydrogen bond is of the 3(10) type between N(3) and O(0), followed by five alpha-helix-type hydrogen bonds. Solution 1H NMR studies in chloroform also favor a helical conformation, with seven solvent-shielded NH groups. Continuous columns are formed by head-to-tail hydrogen bonds between the helical molecules along the helix axis. The absence of polar side chains precludes any lateral hydrogen bonds. Since the peptide crystallizes with one molecule in a triclinic space group, aggregation of the helical columns must necessarily be parallel rather than antiparallel. The packing of the columns is rather inefficient, as indicated by very few good van der Waals' contacts and the occurrence of voids between the molecules.

Interaction of Alpha-tocopherol with diphenylpicryl hydrazyl a means to determine the polarity of the environment around Alpha-tocopherol and its binding with lipids

Alpha-Tocopherol is found to interact with the stable free radical DPPH orders of magnitude faster than ordinary phenols. It is suggested that the high reactivity arises from the coplanarity of the C-O-C framework with the aromatic ring. The rate constant of the reaction of Alpha-tocopherol with DPPH increases progressively with solvent polarity and can be quantitatively related to Kosower's Z parameter. Fatty acid derivatives slow down the reaction with DPPH due to binding with Alpha-tocopherol.

Metabolism of alpha-terpineol by Pseudomonas incognita

Details of the metabolism of alpha-terpineol by Pseudomonas incognita are presented. Degradation of alpha-terpineol by this organism resulted in the formation of a number of acidic and neutral metabolites. Among the acidic metabolites, beta-isopropyl pimelic acid, 1-hydroxy-4-isopropenyl-cyclohexane-1-carboxylic acid, 8-hydroxycumic acid, oleuropeic acid, cumic acid, and p-isopropenyl benzoic acid have been identified. Neutral metabolites identified were limonene, p-cymene-8-ol, 2-hydroxycineole, and uroterpenol. Cell-free extracts prepared from alpha-terpineol adapted cells were shown to convert alpha-terpineol, p-cymene-8-ol, and limonene to oleuropeic acid, 8-hydroxycumic acid, and perillic acid, respectively, in the presence of NADH. The same cell-free extract contained NAD+ -specific dehydrogenase(s) which converted oleuropyl alcohol, p-cymene-7,8-diol, and perillyl alcohol to their corresponding 7-carboxy acids. On the basis of various metabolites isolated from the culture medium, together with the supporting evidence obtained from enzymatic and growth studies, it appears that P. incognita degrades alpha-terpineol by at least three different routes. While one of the pathways seems to operate via oleuropeic acid, a second may be initiated through the aromatization of alpha-terpineol. The third pathway may involve the formation of limonene from alpha-terpineol and its further metabolism.

X-Pro peptides. A theoretical study of the hydrogen bonded conformations of (alpha-aminoisobutyryl-L -prolyl)n sequences

Intramolecularly hydrogen bonded conformations of (Aib-Pro)n sequences have been analysed theoretically. Both 4-1 (C10 and 3-1 (C7 hydrogen bonded regular structures are shown to be stereochemically feasible. Conformational energies for the helical structures have been estimated using classical potential energy methods. Both C10 and C7 conformations have very similar energies. Pyrrolidine ring puckering has a pronounced effect on the energies, and only Cv-endo puckered Pro residues can be accommodated. The theoretical calculations using spectroscopic data suggest that the recently proposed novel 310 helical conformation for benzyloxycarbonyl(Aib-Pro)4-methyl ester is in solution, is indeed energetically and stereochemically favourable.

Pyrolysis and combustion of alpha-cellulose: effect of dihydrazinium phosphate (N2H5)2HPO4

A strip of Whatman filter paper (α-cellulose) dipped in an aqueous solution of dihydrazinium phosphate, (N2H5)2HPO4(DHP), and dried, carbonized without flame when ignited. The observed flame retardancy of DHP on α-cellulose has been studied using TG, DTA and mass spectrometry. Dihydrazinium phosphate appears to catalyze the dehydration of α-cellulose, minimizing the depolymerization which produces flammable tars, with the formation of water and char. Flame retardancy of DHP is compared with that of diammonium phosphate and phosphoric acid.

Hydrophobic channels in crystals of an alpha-aminoisobutyric acid pentapeptide

The crystal structure of the pentapeptide p-toluene-sulfonyl-(α-aminoisobutyryl)5-methyl ester (Tosyl-(Aib)5-OMe) has been determined in the space group PImage . Pentapeptide molecules are folded in the 310 helical conformation and packed together, so as to yield a hydrophobic channel with a minimim diameter of 5.2 �. The channel contains crystallographically disordered material. This structure provides a model for channel formation by hydrophobic peptide aggregates and should prove useful in studies of alamethicin, suzukacillin and related Aib containing membrane channels. Triclinic (PImage ) crystals of the pentapeptide are obtained in the presence of LiClO4 in aqueous methanol, whereas crystallization from methanol alone yields crystals in the space group Pbca. The conformations of the peptide in the two crystal forms are very similar and only the molecular packing is dramatically different.

Allosteric regulation of serine hydroxymethyltransferase from mung bean (Phaseolus aureus)

Serine hydroxymethyltransferase, the first enzyme in the pathway for the interconversion of one carbon compounds was purified from mung bean seedlings by ammonium sulfate fractionation, DEAE-Sephadex, Blue Sepharose CL-6B affinity chromatography and gel filteration on Sephacryl S-200. The specific activity of the enzyme, 0.73 (u mol HCHO formed/min/mg protein) was 104 times larger than the highest value reported hitherto. Saturation of tetrahydrofolate was sigmoid, whereas with serine was hyperbolic, with nH values of 1.9 and 1.0 respectively. Reduced nicotinamide adenine dinucleotide, lysine and methionine decreased, whereas nicotinamide adenine dinucleotide, adenosine 5′-monophosphate and adenosine 5′-triphosphate increased the sigmoidicity. These results suggest that serine hydroxymethyltransferase from mung bean is a regulatory enzyme. H4folate; (±)-L-tetrahydrofolate